Recently, a humidity sensing element the electrostatic capacitance of which varies in accordance with the ambient humidity is developed. When such a sensing element is utilized as a transducer, an electrical circuit which measures the capacitance of the sensing element is required to produce an electrical signal indicative of the capacitance and therefore the humidity. There are several possibilities for such an electrical circuit. For instance, an A.C. bridge or a resonant circuit may be used to measure the capacitance of the sensing element. However, when employing the above mentioned techniques, troublesome steps are required and furthermore, the electrical circuits for achieving the above mentioned techniques are too complex in construction.
Some inventors of the present invention had already proposed in a published British patent specification (Ser. No.: 2,000,292) a new humidity sensing element the electrostatic capacitance of which indicates the ambient humidity and an electrical circuit for displaying the humidity detected by the sensing element by measuring the capacitance of the sensing element.
According to an electrical circuit which measures the capacitance of the sensing element disclosed in the above mentioned British patent application, the sensing element which is substantially functions as a capacitance, is charged by a suitable charging circuit. The voltage across the capacitance, i.e. the sensing element, is detected to measure a period of time required for the voltage to reach a predetermined value. This method is advantageous compared with the above mentioned methods or techniques since the construction of the electrical circuit is simple. However, this electrical circuit also suffers from a drawback that the measured value is apt to include errors due to dielectric loss in the capacitive element. The dielectric loss of the sensing element varies in accordance with ambient temperature, and therefore, accurate measurement of the humidity is not achieved unless the errors due to the dielectric loss is compensated for.